Redskin is not almost exclusively used as a nickname of a football team. It may be where you are at because you may not have that many Native Americans around or that many people who are racist towards Native Americans, but it is absolutely a slur used to describe Native Americans in many places. Just as cracker is for Whites, wetbacks for Hispanics, etc. The more of a said group in an area, the more said terms are used.
You're blowing a lot of smoke. I live in the desert southwest. Several tribes in my area. Nobody uses the word Redskin as a pejorative. The word simply isn't used at all outside the context of team mascots.
Your comparisons fall apart because the words cracker and wetback are actively, currently used as pejoratives; redskins is not, anywhere. As to you saying Redskins is still in use outside the context of football mascots, please, do show an example... anywhere, of such usage, in say, the last 20 years? Good luck with that. In the context of the football team mascot, pretty much the only context in which it is used, it portrays native americans in a positive light.
If you are so sure it is not a pejorative, why don't you start going up to Native Americans and call them Redskins and see what the response is?
It would be absurd to go up to anybody, of any race, and address them using a racial identifier. It would be effrontery, regardless of race. Bottom line, nobody uses Redskin as a group identifier for them. It's usage is limited to mascots, where they are portrayed positively.